1. Field of the Invention
The present invention relates generally to an apparatus for returning blow-by gas collected in a crankcase of an engine to an intake passage so as to reuse it for combustion. More particularly, the present invention relates to a positive crankcase ventilation (PCV) apparatus to forcibly ventilate the crankcase through a gas passage by utilizing a negative pressure in the intake passage.
2. Description of the Related Art
Typically, there are gas leaks between an engine crankcase and its associated combustion chambers through the clearances between cylinders and their piston rings during the compression and power strokes. The leaked gas is known as blow-by gas. Most of the contents of the gas is an intake air-fuel mixture leaking from the combustion chambers in the compression stroke, and the rest is the exhaust gas. Hence, the blow-by gas contains a lot of hydrocarbon. Since the blow-by gas contains a lot of water exhibiting a strong acidity of pH 2 to 3, it has a great effect on deterioration of engine oil and formation of rust inside the engine.
Various devices have been employed to prevent blow-by gas from being released to the atmosphere by returning it to an intake passage. One of the devices is a PCV apparatus to forcibly ventilate the inside of a crankcase through a PCV valve and a gas passage utilizing a vacuum connected to the intake passage. The PCV valve regulates the flow of blow-by gas according to the amount of blow-by gas generated.
Japanese Unexamined Utility Model Registration Publication No. 60-173612 discloses an example of a PCV apparatus. FIG. 7 shows the apparatus of this publication. In the apparatus, a gas passage 51 connects a crankcase 53 of an engine block 52 and a location in the vicinity of a carburetor 55 (including an unillustrated throttle valve). The gas passage 51 allows the blow-by gas to flow therethrough. One end of the gas passage 51 is connected to a location in the vicinity of the carburetor 55 in order to induce a flow of blow-by gas utilizing a negative pressure occurring near the throttle valve. When a PCV valve 56 provided in the gas passage 51 opens, blow-by gas in the crankcase 53 flows to an intake passage 54 through the gas passage 51. The blow-by gas burns in a combustion chamber 57. Water passages 58, 59 for cooling water used for cooling the block 52 contact the gas passage 51 in the vicinity of the PCV valve 56. Cooling water discharged from a water pump (not illustrated) flows through the water passages 58, 59. The cooling water (water heated at the engine block 52) heats the PCV valve 56 and the gas passage 51. This heating prevents both the gas passage 51 and the PCV valve 56 from freezing.
In a modified example of the apparatus described above, a water passage runs along and contacts a gas passage similar to the passage 51. The cooling water flowing through the water passage heats the gas passage in an area extending in the lengthwise direction, and hence the gas passage is more effectively heated.
In some of recent models of vehicles, accessory devices such as a supercharger are disposed away from an engine block. Some of such vehicles employ a separated accessory drive system (SDS) where a throttle valve is located away from an engine block. In such vehicles, the distance between a radiator and the throttle valve is less than the distance between the throttle valve and the engine block. In vehicles which employ the SDS and the PCV apparatus, the throttle valve is located some distance away from the engine block, and hence the gas passage of the PCV apparatus necessarily becomes longer. It has been proposed that the cooling water passage described above be brought in contact this gas passage.
Also, there is a proposal that applies the apparatus disclosed in Japanese Unexamined Utility Model Registration Publication No. 60-173612 to vehicles employing the SDS. This application, however, requires a water passage to contact a longer gas passage over a longer distance for effectively preventing the gas passage from freezing. Moreover, there must be provided two water passages between the engine block and the gas passage. That is, a water feed passage is needed to carry the cooling water discharged from a water pump to the vicinity of the throttle valve. Further, a water return passage is needed which is disposed to run in contact with the gas passage along an area between a location near the throttle valve and a location near the block. Accordingly, the water passage becomes quite long, and the flow rate of the cooling water drops due to the increased flow resistance of the water passage. As a result, the heating of the gas passage may be insufficient, and the gas passage may freeze.